The present invention relates generally to the field of sensor electronics and in particular to a system and method of communicating calibration data from a sensor to a data acquisition system.
Force/Torque (FT) sensors are widely utilized in a broad range of industrial and research applications, including robotics, telerobotics, product testing, orthopedic research, rehabilitation research, haptics research, prosthetics, real-time force control and force feedback, robotic surgery, and many others. FT sensors measure force and/or torque in one or more axes. A six-axis FT sensor, for example, measures force and torque in three orthogonal axes (Fx, Fy, Fz, Tx, Ty and Tz). FT sensors comprise one or more transducers (such as strain gauges) that translate an experienced force or torque into an electrical signal, typically an analog voltage level. The analog signals representing force and torque measurements are often transmitted from the FT sensor, which is often mounted on a robot arm or other restricted space, to a data acquisition (DAQ) system, which may be located some distance from the FT sensor.
To convert a force or torque experienced by the FT sensor from its analog voltage output level to a standard unit, each FT sensor typically has a set of calibration data associated with it. Usually, due to variations in the physical properties of the sensor transducers, each individual FT sensor has a unique set of calibration data; using the calibration data for one FT sensor to calibrate the output of a different sensor—even one from of the same type—will yield erroneous force and torque readings. Conventionally, the calibration data are provided separately from their associated FT sensors, such as on a floppy disk, CD-ROM, or other convenient digital data storage/transfer medium. This practice has the serious disadvantage that, in practice, it is difficult to maintain each FT sensor with its appropriate calibration data, particularly in installations with many different FT sensors and (hence many different sets of calibration data). This leads to the use of incorrect calibration data for many FT sensors. At best, the results are nonsensical, and the correct calibration data must be located and loaded into the data acquisition system, and the force/torque measurements re-taken. At worst, the calibration data used may be very close to, but different from, the correct calibration data. In this case, the force/torque readings are erroneous, but not so egregiously erroneous as to flag suspicion (or trigger range-checking in software) that would indicate the calibration data are amiss.
In addition, even when careful tracking eliminates the confusion between calibration data and individual FT sensors, when an individual FT sensor requires re-calibration or calibration certification, both the FT sensor and its associated calibration must be supplied, requiring careful tracking and coordination throughout these processes as well as during typical use of the FT sensor.
One solution to this problem of physical separation of FT sensors and their associated calibration data is to locate the calibration data within the housing of the FT sensor itself, such as in a ROM, EEPROM, PAL, FPGA, register file, or the like, as is well known in the art. This may result in an FT sensor having analog force/torque reading outputs and digital calibration data, requiring two separate transmission paths (analog and digital) from the FT sensor to the data acquisition system.
One solution to this problem known in the art is to digitize the force/torque outputs at the FT sensor, and transmit only digital data-including both force/torque outputs and calibration data-to a data analysis system. However, this solution presents several drawbacks. FT sensors are often designed to be as compact as possible, and the inclusion of analog-to-digital converter (ADC) circuits in the FT sensor housing increases its size. In many applications, FT sensors must be rugged, and the ADC electronics may adversely impact ruggedness and reliability. Additionally, in many applications, precise quantization of the force/torque readings is required, necessitating sophisticated data acquisition (DAQ) electronics that may be updated several times over the life of the FT sensor. In such applications, a superior long-term solution is for the FT sensor to output analog values, and receive them with a DAQ card of arbitrary complexity and sophistication. An additional benefit of this solution is that a variety of DAQ cards are commercially available for each generation of computer interface bus, such as EISA, MCA, PCI, S-Bus, etc. Thus, the data analysis system can easily grow in sophistication without sacrificing the investment in a particular set of FT sensors. However, the problem of the association of digital calibration data with the analog FT sensor, and the transmission of both to a data acquisition system, remains.